1. Field of the Invention
The invention relates to compositions of carbide-containing and oxycarbide-containing nanorods, carbon nanotubes including carbide and/or oxycarbide compounds, rigid porous structures including these compositions, and methods of making and using the same. More specifically, the invention relates to rigid three dimensional structures comprising carbide and/or oxycarbide-containing nanorods or carbon nanotubes bearing carbides and oxycarbides, having high surface areas and porosities, low bulk densities, substantially no micropores and increased crush strengths. The invention also relates to using the compositions and the rigid porous structures including these compositions as catalysts and catalyst supports, in heterogeneous catalytic reactions frequently encountered in petrochemical and refining processes.
2. Description of the Related Art
Heterogeneous catalytic reactions are widely used in chemical processes in the petroleum, petrochemical and chemical industries. Such reactions are commonly performed with the reactant(s) and product(s) in the fluid phase and the catalyst in the solid phase. In heterogeneous catalytic reactions, the reaction occurs at the interface between the phases, i.e., the interface between the fluid phase of the reactant(s) and product(s) and the solid phase of the supported catalyst. Hence, the properties of the surface of a heterogeneous supported catalyst are important factors in the effective use of the catalyst. Specifically, the surface area of the active catalyst, as supported, and the accessibility of that surface area to reactant adsorption and product desorption are important. These factors affect the activity of the catalyst, i.e., the rate of conversion of reactants to products. The chemical purity of the catalyst and the catalyst support have an important effect on the selectivity of the catalyst, i.e., the degree to which the catalyst produces one product from among several products and the life of the catalyst.
Generally catalytic activity is proportional to catalyst surface area. Therefore, a high specific area is desirable. However, the surface area should be accessible to reactants and products as well as to heat flow. The chemisorption of a reactant by a catalyst surface is preceded by the diffusion of that reactant through the internal structure of the catalyst.
Since the active catalyst compounds are often supported on the internal structure of a support, the accessibility of the internal structure of a support material to reactant(s), product(s) and heat flow is important. Accessibility is measured by porosity and pore size distribution. Activated carbons and charcoals used as catalyst supports may have surface areas of about a thousand square meters per gram and porosities of less than 1 ml/gm. However, much of this surface area and porosity, as much as 50%, and often more, is associated with micropores, i.e., pores with pore diameters of 2 nm or less. These pores can be inaccessible because of diffusion limitations. They are easily plugged and thereby deactivated. Thus, high porosity material where the pores are mainly in the mesopore region, i.e., greater than 2 nm or macropore region, i.e., greater than 50 nm, ranges are most desirable.
It is also important that self-supported catalysts and supported catalysts not fracture or attrit during use because such fragments may become entrained in the reaction stream and must then be separated from the reaction mixture. The cost of replacing attritted catalyst, the cost of separating it from the reaction mixture and the risk of contaminating the product are all burdens upon the process. In slurry phase, e.g. where the solid supported catalyst is filtered from the process stream and recycled to the reaction zone, the fines may plug the filters and disrupt the process. It is also important that a catalyst, at the very least, minimize its contribution to the chemical contamination of reactant(s) and product(s). In the case of a catalyst support, this is even more important since the support is a potential source of contamination both to the catalyst it supports and to the chemical process. Further, some catalysts are particularly sensitive to contamination that can either promote unwanted competing reactions, i.e., affect its selectivity, or render the catalyst ineffective, i.e., “poison” it. For example, charcoal and commercial graphites or carbons made from petroleum residues usually contain trace amounts of sulfur or nitrogen as well as metals common to biological systems and may be undesirable for that reason.
Since the 1970s carbon nanofibers or nanotubes have been identified as materials of interest for use as catalysts and catalyst supports. Carbon nanotubes exist in a variety of forms and have been prepared through the catalytic decomposition of various carbon-containing gases at metal surfaces. Nanofibers such as fibrils, bucky tubes and nanotubes are distinguishable from continuous carbon fibers commercially available as reinforcement materials. In contrast to nanofibers, which have, desirably large, but unavoidably finite aspect ratios, continuous carbon fibers have aspect ratios (L/D) of at least 104 and often 106 or more. The diameter of continuous fibers is also far larger than that of nanofibers, being always greater than 1 μm and typically 5 μm to 7 μm.
U.S. Pat. No. 5,576,466 to Ledoux et al. discloses a process for isomerizing straight chain hydrocarbons having at least 7 carbon atoms using catalysts which include molybdenum compounds whose active surface consists of molybdenum carbide which is partially oxidized to form one or more oxycarbides. Ledoux et al. disclose several ways of obtaining an oxycarbide phase on molybdenum carbide. These methods require the formation of molybdenum carbides by reacting gaseous compounds of molybdenum metal with charcoal at temperatures between 900° C. and 1400° C. These are energy intensive processes. Moreover, the resulting molybdenum carbides have many drawbacks similar to catalysts prepared with charcoal. For example, much of the surface area and porosity of the catalysts is associated with micropores and as such these catalysts are easily plugged and thereby deactivated.
While activated charcoals and other materials have been used as catalysts and catalyst supports, none have heretofore had all of the requisite qualities of high surface area, porosity, pore size distribution, resistance to attrition and purity for the conduct of a variety of selected petrochemical and refining processes. Although many of these materials have high surface area, much of the surface area is in the form of inaccessible micropores.
It would therefore be desirable to provide a family of carbide-containing and oxycarbide containing catalysts that have highly accessible surface area, high porosity, and attrition resistance, and which are substantially micropore free, highly active, highly selective and are capable of extended use with no significant deactivation.
Nanofiber mats, assemblages and aggregates have been previously produced to take advantage of the increased surface area per gram achieved using extremely thin diameter fibers. These structures are typically composed of a plurality of intertwined or intermeshed nanotubes.